Various types of biometric systems are used more and more in order to provide for increased security and/or enhanced user convenience. In particular, fingerprint sensing systems have been adopted in, for example, consumer electronic devices, thanks to their small form factor, high performance and user acceptance.
To provide for a low energy consumption of the fingerprint sensing system, the fingerprint sensing system should only operate when there is a finger on the fingerprint sensing device comprised in the fingerprint sensing system. On the other hand, the fingerprint sensing system may be required to independently determine whether or not a finger is present on the sensing device and to sense the fingerprint pattern of the finger when the finger is placed on the sensing device.
U.S. Pat. No. 8,031,046 discloses a finger sensing device in which finger sensing electrodes may be bussed together and connected to a bussed electrode amplifier which is typically maintained on. A sample and hold and decoder circuit is connected to the bussed electrode amplifiers and may output an average power from the bussed group of electrodes that is compared with a threshold level. If the average power is higher than the threshold level, it is determined that the finger is present, and a fingerprint image is acquired.
Although the solution proposed by U.S. Pat. No. 8,031,046 seems to provide for a rather energy-efficient finger detect functionality, there still appears to be room for improvement.